Thermal Measurement

THERMAL MEASUREMENTS

Every material used in an envelope assembly has fundamental physical properties. These properties determine their energy performance like conductivity and resistance. Understanding these properties helps us to choose the right materials for heat flows.

1. Thermal Conductivity (K)

 It is a material's ability to conduct heat.

Each material has a characteristic rate at which heat will flow through it. The faster heat flows through a material, the more conductive it is.

It is a material property given for homogeneous solids under steady state conditions.

It is given by the following equation:

K= QL/A∆T

where

Q- the resultant heat flow

K- thermal conductivity of the material (W/mK) or Wm ^-1 K^-1

A -  surface area through which the heat flows (m²)

∆T  - temperature difference between the warm and cold sides of the material (K)

L - thickness of the material (m)

2. Thermal Conductance (C)

It is the thermal conductivity per unit area for a specified thickness. It is used for standard building materials.

In basic building materials, heat flow is usually measured by thermal conductance (C), not thermal conductivity. Thermal conductance is a material's thermal conductivity per unit area. It has unit of  W m^-2 K ^-1

3. U - Factor (U)

It is the overall conductance of a building element. It is used for layered building assemblies.

In layered assemblies, thermal conductances are combined into a single number called the "U-factor" (or sometimes the "U-value").

U is the overall coefficient of thermal transmittance. It is expressed in terms of W/(m² K). The thermal conductance is used for specific material, whereas U -factor is used for a specific assembly. Lower U-factors mean less conduction, which means better insulation.

For instance, the overall U-factor of a window includes the thermal conductances of the glass panes, air inside, framing material, etc.

4. Thermal Resistance (R value = 1/U)

It is a material's ability to resist heat flow.

It is designated as R (R value), thermal resistance indicates how effective any material is as an insulator.

The reciprocal of thermal conductance is known as thermal bewar resistance R. It is measured in SI unit of m² K/W.

• For a homogeneous material such as wood, doubling the thickness will double the R value. These values are not specified for assemblies of materials. U - factors are used for assemblies.

Thermal insulation which prevents heat flow through the building envelope, is often measured by its R-value. A higher R - value indicates a better thermal insulating performance.

Calculating the overall U-factor starts with adding thermal resistances. The U-factor is the reciprocal of sum of resistances of all the material, U= 1/Σ R.

5. Thermal Mass

Thermal mass is a material's resistance to change in temperature as heat is added or removed. It is a key factor in dynamic heat transfer interactions within a building.

The four factors to understand are: density, specific heat, thermal capacity, and thermal lag.

Density

Dense materials usually store more heat.

Density is the mass of a material per unit volume. In the SI system, it is given as kg/m³. For a fixed volume of material, greater density will permit the storage of more heat.

Specific Heat

High specific heat requires a lot of energy to change the temperature.

Stud Specific heat is a measure of the amount of heat required to raise the temperature of given mass of material by 1°K. In the SI system, it is expressed as J/kg K.

It takes less energy input to raise the temperature of a low-specific-heat material than that of a high-specific-heat material.

For instance, one gram of water requires one calorie of heat energy to rise 1°C. Water has a high heat capacity and therefore is sometimes used as thermal mass in buildings.

Thermal Capacity (Thermal Mass)

Density × Specific Heat = How much heat can be stored per unit volume

Thermal capacity is an indicator of the ability of a material to store heat per unit volume.

The greater the thermal capacity of a material, the more heat it can store in a given volume per degree of temperature increase.

Thermal capacity for a material is obtained by taking the product of density and specific heat. Unit is J/K.

Higher thermal capacity can (but will not always) reduce heat flow from the outside to the inside environment by storing the heat within the material.

6. Insulation and Window Performance

Some of the highest thermal energy losses from a building are through windows and poorly insulated walls. (Fig. 1.18)

There is currently no accurate way of monitoring how well a building's thermal insulative materials are performing.

Highly expensive infrared cameras are typically used to compare the efficiencies of different windows and insulation but reflectivity of the surfaces dramatically affects their accuracy.

IR cameras are also limited because they only show relative maps of temperature and not quantitative heat loss. These IR instruments often give false information about the thermal envelope of buildings and how well the insulation is performing.

Unlike typical thermal sensors that only give values of temperature at certain locations, heat flux sensors directly IsmedT measure thermal energy through surfaces.

Heat flux sensors can be used to determine R-values of windows, walls, and other building materials. (Fig. 1.19)

Measured heat flux values from heat flux sensors can be correlated to total heat loss from a building.